* Engine size and type: A larger engine will generally have lower vacuum than a smaller one. V6 and V8 engines typically have lower vacuum than inline-4s. The design of the intake manifold also plays a role.
* Engine condition: Leaks in the intake manifold, vacuum lines, or the PCV system will drastically reduce vacuum. Worn piston rings or valve guides can also lower vacuum.
* Altitude: Higher altitudes mean lower atmospheric pressure, resulting in lower vacuum readings.
* Engine load: Even at idle, slight variations in load (e.g., accessories turning on) will affect vacuum.
Instead of a specific number, a better approach is to compare your reading to the specifications for your specific engine. Your owner's manual or a reliable repair manual for your vehicle model and year should provide the expected vacuum range at idle.
Generally, you'll see vacuum readings between 15 and 22 inches of mercury (inHg) at idle for many gasoline engines. Readings significantly below this range often suggest a problem. However, always consult your vehicle's specifications. Using a vacuum gauge and comparing your reading to the specifications is the best way to diagnose potential problems.
